Many of the gram-negative flora of the gastrointestinal tract of mammals belong to a group called coliforms, since they are found in large numbers in the colon and resemble the characteristic species Escherichia coli. Coliforms can be differentiated from other G- rods based on their ability to ferment the milk sugar lactose. Non-coliform rods such as Salmonella typhimurium, a pathogen normally found in the gastrointestinal tract of reptiles and birds, and Proteus mirabilis, an opportunistic pathogen of the urinary tract, lack the enzymes necessary to metabolize this sugar. By adding lactose and one or more pH indicators to a selective medium for G- rods, we can easily differentiate between the two groups.
agar is a medium that contains bile salts and crystal violet as its selective
agents. These inhibit the growth of most gram-positives. It
also contains lactose and a neutral red indicator that turns pink in the
presence of the metabolic by-products of lactose fermentation. In
this image, the two lactose-fermenting coliforms E. coli and E.
aerogenes have produced the characteristic pink color, while S.
typhimurium and P. mirabilis, the two non-coliforms have not.
is also used to select for an differentiate between groups of gram-negative
rods. Hektoen agar contains bile salts, lactose, acid fuschin and
brom thymol blue indicators. It also contains sodium thiosulfate
and ferric ammonium citrate, used to determine hydrogen sulfide (H2S)
production. Coliform rods will produce colonies that range in color
from yellow to orange-yellow, while non-coliform colonies remain the green
color of the agar itself. Organisms such as S. typhimurium
have the ability to utilize sulfate as a final electron and hydrogen acceptor
during anaerobic respiration, producing H2S as a by-product.
This undergoes an exchange reaction with ferric ammonium citrate, producing
ferric sulfate (FeS), a black colored precipitate. Given time, the
green colonies of S. typhimurium will develop a black center, thus
are sometimes referred to as "fish eye" colonies.